SB 608 
.T75 L5 
Copy 1 



PHOMA ROT OF TOMATOES 



GEORGE K. K. LINK 

and 

F. C. MEIER 

Pathologists 




UNITED STATES DEPARTMENT OF AGRICULTURE 
DEPARTMENT CIRCULAR 219 



Contribution from the Rureau of Plant Industry 

(OflBce of Cotton, Truck, and Forage Crop Disease Investigations) 

WM. A. TAYLOR, Chief 



Waahington, D. C. 



May, 1922 



tSHINQTON raOVERNMENTPRINTINQ OFFICE : Ittt 



Monoevophk 






PHOMA ROT OF TOMATOES 



CONTENTS. 



Page. 



What is Phoma rot of the tomato? 

What is the effect of Phoma rot on 
the fruit? 

What is the effect of Phoma blight 
on the plant? 

Where and when do the rot and 
blight occur? 

Where and how does the blight start 
in the field? __ 



Page. 



Where and how does the fungus get 
on the fruit? 

What favors the start and develop- 
ment of the fungus? 

Does Phoma rot develop and spread 
after the fruit is picked? 

What should be done with the dis- 
eased fruit? 

How can the disease be controlled 
in the field and in transit? 



WHAT IS PHOMA ROT OF THE TOMATO? 

PHOMA ROT of tomato fruits, also known as stem-rot, or black- 
rot, is a disease caused by a mold, or fungus.^ This fungus also 
attacks the leaves and stems of tomato vines, causing the disease 
known as Phoma leaf -blight or stem-blight. 

WHAT IS THE EFFECT OF PHOMA ROT ON THE FRUIT? 

Phoma rot makes affected fruit unsalable, or at least lowers its 
selling price, and often leads to its complete decay. 

The fungus attacks both green and ripening fruit. In its earliest 
stages, Phoma rot of green fruit is marked by minute, slightly sunken 
spots. As these spots enlarge they develop brown or black borders 
with slightly lighter centers which become dotted with black pimple- 
like bodies (pycnidia) in which the seedlike bodies, or spores, of the 
fungus are borne. 

^ This fungus is known as Phoma def<tructiv(i. A fungus (plural, fungi) is a small, 
simple plant, which lacks the green color found in more complex plants, such as our 
orchard, field, and garden crops. Molds, yeast, toadstools, and mushrooms are good 
examples of fungi. Fungi can not make their own food, as green plants do, and many 
of them obtain it from living plants or their dead remains. Those fungi which obtain 
their food and energy from living plants are known as parasites, while the plants upon 
which they grow and feed are known as their hosts. Such fungi usually cause harmful 
changes in the structure, composition, and actiAities of their hosts, which are known as 
lesions. Such changes constitute di.sease. Thus, in Phoma rot of tomato fruits the 
fungus PJwma destrnctiva is the parasite which attacks and feeds upon its host, the 
tomato, causing spots or lesions, which are manifestations of the disease. 

4768—22 1 



2 Department Circular 219, U. S. Dept. of Agriculture. 

On ripe fruit the earliest stages of the rot are marked by water- 
soaked concave spots. As these enlarge, their advancing edge remains 
slightly sunken, slightly discolored, and water-soaked. Their centers 
become brown to charry black, leathery and firm, and dotted with 
the black pimplelike fruiting bodies of the fungus. Under moist 
conditions spores in the form of slimy whitish masses ooze out of these 
fruiting bodies so abundantly that the fruiting bodies become com- 
pletely covered and the center of the spot appears whitish and slimy. 
The spots increase indefinitely and rapidly in size, but remain more or 
less circular and concave. The underlying affected parts are brown 
or black and remain firm until the spots become very large or infected 
with organisms that cause soft rots. The dry crustlike center of. the 
spot often cracks, and a grayish white growth of the fungus may 
develop in the crevices. Frequently numerous small spots merge to 
form a large one. 

The spots occur most frequently at the stem end of the fruit, where 
the fungus enters through growth cracks or breaks made when the 
fruit is picked ; they are not confined to the stem end, however, since 
the fungus may enter the fruit through breaks in the skin made by 
insects, by cracking, or by other diseases, such as nailhead spot. 

WHAT IS THE EFFECT OF PHOMA BLIGHT ON THE PLANT? 

Phoma blight consists of spots on the leaves, leaf petioles, and the 
stems of the vines. (PI. I.) Spotted leaves dry out and are shed 
prematurely, thus causing a reduction of leaf surface, which lowers 
the vigor of the plants and frequently causes heavy loss through 
decrease in yield. The disease spreads to the fruit from these leaf 
and stem spots, appearing on the fruit in the field and in transit as 
the rot previously described. 

In the early stages the spots on the leaves are mereiy small discol- 
ored areas, later turning brown or black, which are definite in outline 
and irregular in shape. These spots do not change color as they 
become older, but do enlarge, and often several merge into a large 
blotch of definite outline but irregular shape. They frequently start 
along the leaf margins. On petioles and stems the diseased areas 
appear as streaks or blotches. The dead areas on leaves, stems, and 
petioles dry out and shrivel, and on them appear, singly or in groups, 
the small black pimplelike fruiting bodies in which the spores of the 
fungus are borne. 

WHERE AND WHEN DO THE ROT AND BLIGHT OCCUR? 

Phoma blight and rot occur most frecjuently in southern winter- 
grown tomatoes, that is, in the crops of California, Cuba, Florida, 



Cir. 219, U. S. Dept. of Agriculture. 



Plate I. 




Phoma Rot of the Tomato. 

The disease attacks tomato foliage as well as fruit. Fruits affected with growth cracks, 
shown in the tomato just above the basket, are particularly susceptible. 



Phoma Rot of Tomatoes. 3 

and Mexico. It has also been noted in Texas, Mississippi, Tennessee, 
and in northern fields. 

WHERE AND HOW DOES THE BLIGHT START IN THE FIELD? 

The growing plant gets the disease from the soil where the fungus 
may overwinter, possibly from seed extracted from tomatoes har- 
vested in Phoma-infested fields, or possibly from near-by wild or 
cultivated plants of the potato family infested with the fungus. 

WHERE AND HOW DOES THE FUNGUS GET ON THE FRUIT? 

As pointed out previously, the spores, or seedlike bodies of the 
fungus, are produced in the black pimplelike bodies which appear 
on the diseased spots of affected vines. These spores may be directly 
carried to the fruit by rain, wind, insects, and man, or they may 
become lodged in the soil and subsequently spattered on the fruit by 
rain or carried to it by wind or insects. They probably are also car- 
ried to fruit in picking and packing operations. 

Fruit which carries these spores on its surface is said to be con- 
taminated, and it is probable that most fruit picked in a diseased field 
has spores on it, especially if the field is widely and severely affected. 

A fruit may carry many spores of this fungus on its surface and 
still be perfectly sound or healthy and remain so. Before the disease 
can develop in a contaminated fruit, either in the field or after pick- 
ing, infection must take place; that is, the spores must sprout, or 
germinate, and the resulting fungus must enter the tissues of the 
fruit and establish itself there. Laboratory and field studies indi- 
cate that the fungus can not penetrate the uninjured skin of tomato 
fruits; hence, only tomatoes the skin of which is broken by insects, 
bruising, natural cracking, or other diseases can become infected. 
For this reason Phoma rot occurs most frequently on the stem end of 
fruits, either at growth cracks or in the fruit-stem scar. Wounds, 
however, need not be so large as to be visible in order to permit the 
entrance of this fungus. It is probable that infection of contami- 
nated fruit is not restricted to the field, but that it also takes place 
in packing houses and during transit. 

WHAT FAVORS THE START AND DEVELOPMENT OF THE 

FUNGUS? 

The fungus can not grow if the temperature is either below 42° or 
above 90° F., and its spores can not germinate if they are not sup- 
plied with moisture. Consequently the disease can not develop and 
spread in the field unless dews or rainy weather occur. The fungus 
grows best when the temperature is about 83° F. 



4 Department Circular 219, U. S. Dept. of Agriculture. 

As already stated, the fungus appaiiently can not penetrate the skin 
of the tomato. As a consequence, contaminated fruit can not become 
infected unless its skin is broken, even though temperature and 
moisture conditions are favorable for the growth of the fungus. 

Following infection some time must elapse before visible symptoms 
of the disease appear. It may happen, therefore, that fruit is 
packed which looks sound but which has become infected and will 
develop visible signs of the disease later if temperature and moisture 
conditions are favorable for the fungus. 

Experiments show that at least 4 days must elapse before any signs 
of the disease become visible in wounded contaminated fruit. After 
10 days the spots become distinct, and after 3 weeks the black pimple- 
like fruiting bodies appear. It was found that at 64° F. in a humid 
atmosphere, spots may reach a diameter of 1 to 2 inches in 18 days. 
The spots enlarge more rapidly in ripening than in green fruit. 
For example, it was found that spots on ripe fruit were one-fifth to 
three-fifths of an inch in diameter after 10 days, whereas the spots 
on green fruit kept under the same conditions were only one-fifth as 
large. 

Under the most favorable conditions for the fungus, high hu- 
midity and air temperature of about 80° F.. the disease becomes 
visible on infected leaves in 3 to 5 days as slightly discolored spots. 
In case of severe infection these spots give the leaves a speckled ap- 
pearance. About 8 days after infection the spots become black and 
are about one-tenth to two-fifths of an inch in size. 

DOES PHOMA ROT DEVELOP AND SPREAD AFTER THE FRUIT 

IS PICKED? 

Every day's delay in getting infected fruit to the consumer 
increases the possibility of loss. Fruit picked in an infected field 
but appearing sound when wrapped, even though actually infected, 
may show spots in 4 or 5 days. Furthermore, fruit which comes 
from a disease-free field but which is packed or repacked in places 
where diseased fruit or fruit from diseased fields is handled is likely 
to become infected and develop the disease in transit or in the 
ripening rooms. 

WHAT SHOULD BE DONE WITH THE DISEASED FRUIT? 

Spotted fruit, even though unsightly, is fit for food provided 
the diseased parts are cut away. It should be disposed of as soon 
as possible, however, to avoid greater loss, since the spots enlarge 
rapidly and also open the way for other organisms of decay, which 
may rot the fruit completely in a very short time. 



Phoma Rot of Tomatoes. ' 5 

HOW CAN THE DISEASE BE CONTROLLED IN THE FIELD AND IN 

TRANSIT? 

No satisfactory effective measures for the control of the disease 
in the field have been determined. Crop rotation and the planting 
of disease-free seed probably are advisable. 

Since the fungus can enter fruit only through wounds, it is highly 
desirable that fruit be handled very carefully to avoid breaking the 
skin. 

The only effective way to prevent losses in transit or in the 
marketing processes is to handle only fruit grown in disease-free 
fields and not packed or repacked in places where Phoma-infected 
fruit is handled. 



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